Shock struts are used in a wide variety of vehicle suspension systems for controlling motion of the vehicle and its tires with respect to the ground and for reducing transmission of transient forces from the ground to the vehicle. Shock struts are a common and necessary component in most aircraft landing gear systems. The shock struts used in aircraft landing gear systems are subjected to more demanding performance requirements and operational conditions because the shock strut must control motion of the landing gear and absorb, damp and react forces or loads imposed on the landing gear during landing, taxiing, takeoff, maintenance and other operational conditions.
The shock strut generally accomplishes these functions by compressing a fluid within a sealed chamber formed by hollow, telescoping cylinders. The fluid generally includes both a gas and a liquid, in which the liquid may take the form of hydraulic fluid or oil. One type of shock strut is generally referred to as an “air-over-oil” shock strut where a trapped volume of gas is compressed and a volume of the liquid is metered through an orifice in one of the telescoping cylinders when the shock strut is axially or longitudinally compressed. The gas operates as an energy storage device, similar conceptually to a mechanical a spring, so that upon termination of a compressing force the shock strut returns to its original length. Shock struts also dissipate energy by passing the liquid through the orifice so that as the shock strut is compressed or extended, its rate of motion is limited by the damping action that occurs from the liquid being forced through the orifice.
Landing gear systems for aircraft are qualified and certified based upon various operational loads and shock strut stroke parameters. These loads typically include a limit load for both structural certification and design validation. Using the limit load, an ultimate load factor may be applied to achieve an acceptable and often required maximum structural capability. The ultimate load factor is typically 1.5 times the limit load, but may have other values or additional factors of safety based on design or certification criteria. By way of example, the aforementioned limit and ultimate load requirements are defined in the Federal Aviation Regulations for commercial aircraft that will operate within the boundaries of the United States or its territories.
For the shock strut in particular, these load requirements general dictate the structural arrangement of the shock strut components based upon a combination of bending and column stability. For example, structural sizing of at least one or more of the shock strut components may be driven by the in-stroke characteristics of the shock strut in combination with applied vertical loads, applied drag loads, and applied side loads. In view of the load requirements and the commonly applied structural sizing analyses, it is often a substantial challenge to reduce an overall weight of various landing gear systems.